This invention relates to the provision of coatings on the surface of glass fibres, particularly, though not necessarily exclusively, on the surface of freshly drawn optical fibre that has been prepared by a method involving the vapour deposition of silica.
It is well known that the initial tensile strength of freshly drawn silica optical fibre deteriorates very significantly if the surface of the fibre is not almost immediately protected. For this reason much silica optical fibre is provided with a plastics protective coating using coating apparatus that is located immediately beneath the fibre drawing apparatus and is operated on-line with the drawing operation. For many applications the protection provided for the pristine glass surface of the fibre by a plastics protective coating provided in this way is satisfactory. For some applications, however, it is not. Typically this is because plastics coatings are found insufficiently impermeable to water, and the presence of water at the glass fibre surface is particularly liable to promote a reduction in tensile strength through the agency of stress-corrosion. Additionally it may be found that plastics protective coatings provide inadequate resistance to permeation by hydrogen. For many applications a barrier to penetration by hydrogen is useful primarily because of the relatively strong optical absorption bands present at inconvenient regions of the optical spectrum that are associated with the presence of hydrogen.
In U.S. Pat. No. 4,183,621, to which attention is directed, there is described a construction of coated optical fibre in which the glass surface of a silica optical fibre is provided with a carbon barrier layer impervious to moisture. Methods of coating that are suggested comprise passing the fibre through a colloidal suspension of carbon particles and then heating the deposited carbon, plasma coating, chemical vapour deposition, vacuum evaporation, and pyrolysis in a reducing or inert atmosphere of a hydrocarbon such as methane.
U.S. Pat. No. 4,575,463 also relates to the provision of hermetic coatings on optical fibres, and is particularly concerned with the provision of heterogeneously nucleated chemical vapour deposition coatings where the deposition reaction is initiated at the surface of the fibre as the result of its high temperature. This high temperature may be provided by locating the coating deposition chamber relatively close beneath the fibre drawing furnace so that the fibre is caused to enter the deposition chamber very shortly after it has left the drawing furnace. However, as observed in European Patent Specification No. 0,308,143 A, U.S. Pat. No. 4,575,463 does not specifically mention the deposition of hermetic coatings made of carbon. EP 0,308,143 A states that a number of different hydrocarbons can be used in a heterogeneously nucleated chemical vapour deposition reaction to produce a carbon coating upon an optical fibre, particularly exemplifying the use of acetylene for this purpose.
It is highly desirable that such a carbon layer is appled to freshly drawn fibre before it touches any solid surface since contact with such surfaces is liable to impair fibre strength. If the fibre is not to touch any solid surface prior to being coated, the coating of the fibre freshly drawn from fibre preform must take place in the space existing between the bottom of the drawing zone and the first place that the fibre changes direction, either as the result of becoming wound upon the surface of a take-up drum, or as the result of being deflected round some form of guide pulley. Part of this space may also be required for the application and curing of a plastics protective coating for the fibre. The available space for coating with carbon is thus typically strictly limited, and this in turn means that coating efficiency is a critical parameter if a sufficient depth of impermeable carbon is to be deposited in the space available. In this context it is to be noted that the dwell time of any stretch of fibre within the space available for coating is reduced as the drawing speed is increased. Thus,in the provision of an impervious carbon coating, a problem is liable to be encountered in achieving a sufficient depth of coating within the time and space permitted. Surprisingly it was found that the rate of deposition, and hence the thickness of coating provided by a particular set of deposition conditions depended to a marked extent upon the hydrocarbon chosen as the source material of the carbon coating. In a particular instance, using a drawing speed of 20 metres per minute, a coating thickness of less than 1.6 nm was achieved using methane in a reactor 600 mm long. This is to be contrasted with the obtaining of a coating thickness of 108 nm when a particular hydrocarbon was substituted for the methane without changing any of the other deposition conditions.